Thorough descriptio-rays about interaction of X-rays with matter

1. Basic interaction
of X- Ray with
matter
By Chirag Singh Dogra

2. 5 types
1. Coherent Scattering
2. Photoelectric effect
3. Compton scattering
4. Pair production
5. Photodisintegration

3. ● When photon get absorbed they get compleaterly
removed from X-ray beam and ceases to exist.
● When photon are scattered they are deflected into
random course, & no longer contain useful information &
result into noise .

4. ● Noise created by scatter radiation is called film fog.

5. Coherent Scattering
● Photon undergoes change in direction without change in
wavelength.
● “Unmodified scattering”
● Thomson scattering - Single electron is involved in
interaction.
● Rayleigh scattering - Scattering results from cooperative
interaction with all the electron of an atom.

6. Coherent scattering
● No transfer of energy takes place.
● Only type of interaction which doesn’t cause ionization.
● Sets electron into vibration of same frequency as that of
the incident photon.

7. Photoelectric effect
● An incident photon with little more energy than the
binding energy of K shell electrons encounters with that
electron and removes it from its orbit.
● Photon dissappears
● Photon’s energy = Binding energy of k shell e + Kinetic
energy of ejected electron

8. Photoelectric effect
● Vaccancy in K shell is filled by an electron from L shell or
M shell ( Occasionally) or free e from same or another
atom ( Rarely)
● As e drops into K shell it emits energy in form of X-ray
photon ( Characterstic radiation) . Energy of photon =
Difference in the energy of two consecutive shells.

9. Photoelectric effect
● Probability of occurence governed by 3 factor :-
1. Incident photon must have sufficient energy to overcome
binding energy of e
2. Photoelectric effect is most likely to occur when photon’s
energy and binding energy are nearly same
Photoelectric effect ∝ 1/(energy)3

10. Photoelectric effect
3. The tighter the e is bound in its orbit, more likely it is to be
involved in the photoelectric reaction
Photoelectric effect ∝( Atomic number)3
● Photoelectric effect cannot take place with a free e .

11. Photoelectric effect
● Application to diagnostic radiology:-
1. It produces radiographic image of excellent quality
because it doesn’t produce scattered radiation & it
enhances natural tissue contrast.
2. Patient receive more radiation from photoelectric effect
than from any other type of interaction.

12. Compton scattering
● Almost all the scatter radiation that we encounter in
diagnostic radiology comes from Compton scattering.
● An photon with relatively high energy strikes a free outer
shell electron ejecting it from its orbit.
● Photon always retains part of its original energy.
● Reaction produces ion pair, positive atom, negative
electron.

13. Compton scattering
● Energy of incident photon distributed in two ways :
Kinetic energy of recoil electron & part of it is retained by
deflected photon.
● No energy is needed to free electron from its shell (unlike
photoelectric effect).
● Amount of energy photon retains depends on the initial
energy & angle of deflection of recoil electron.

15. Compton scattering
● Amount of energy imparted to recoil electron ∝ angle of
photon deflection.
● Photon never gives up all its energy.
● Higher the energy of photon, the more difficult they are to
deflect.

16. Compton scattering
● Photon with narrow angle of deflection, retain almost all
of their original energy & have excellent chances of
reaching X-ray film and producing fog, they are too
difficult to remove by filters because they are too
energetic and they are difficult to remove by grid because
of their low angle of deflection.

17. Compton scattering
● Probability of occurence
Number of reaction decreases with increase in photon
energy.

18. Pair production
● High energy photon interact with nucleus of atom, its
energy get converted into matter in form of 2 particles ;
Electron and Positron.
● Mass of one electron= 0.52 MeV
● Does not takes place with photon energy less than 1.02
MeV

19. Photodisintegration
● Part of atom’s nucleus is ejected by high energy photon.
● Ejected part may be Nuetron, An alpha particle, or cluster
of particles.
● Photon must have enough energy to overcome binding
energy of nucleus.
● 7-15 MeV

20. Thank You